window.onload = function() {

    var canvas = document.getElementById("canvas");
    var ctx = canvas.getContext("2d");

    var pi = Math.PI;

    var centerX, centerY;
    var part_num = 2000;

    var mousedown = false;
    var X, Y;
    /*===========================================================================*/

    /*===========================================================================*/
    var P = [];
    var part = function(x, y, vx, vy, r, red, green, blue, alpha, col) {
        this.x = x;
        this.y = y;
        this.vx = vx;
        this.vy = vy;
        this.r = r;
        this.red = red;
        this.green = green;
        this.blue = blue;
        this.alpha = alpha;
        this.col = col;
    };

    function rand(min, max) {
        return Math.random() * (max - min) + min;
    }

    function dist(dx, dy) {
        return Math.sqrt(dx * dx + dy * dy);
    }

    function size() {
        canvas.width = window.innerWidth;
        canvas.height = window.innerHeight;

        centerX = canvas.width / 2;
        centerY = canvas.height / 2;
    }
    size();
    X = centerX;
    Y = centerY;

    function init() {
        var x, y, vx, vy, r, red, green, blue, alpha, col;
        for (var i = 0; i < part_num; i++) {
            x = rand(0, canvas.width);
            y = rand(0, canvas.height);
            vx = rand(-1, 1);
            vy = rand(-1, 1);
            r = rand(1, 3);
            red = Math.round(rand(150, 200));
            green = Math.round(rand(100, 255));
            blue = Math.round(rand(180, 255));
            alpha = 1;
            col = "rgba(" + red + "," + green + "," + blue + "," + alpha + ")";

            P.push(new part(x, y, vx, vy, r, red, green, blue, alpha, col));
        }
    }

    function bg() {
        ctx.fillStyle = "rgba(25,25,30,1)";
        ctx.fillRect(0, 0, canvas.width, canvas.height);
        //ctx.clearRect(0, 0, canvas.width, canvas.height);
    }

    function bounce(b) {

        if (b.x < b.r) {
            b.x = b.r;
            b.vx *= -1;
        }
        if (b.x > canvas.width - b.r) {
            b.x = canvas.width - b.r;
            b.vx *= -1;
        }

        if (b.y - b.r < 0) {
            b.y = b.r;
            b.vy *= -1;
        }
        if (b.y > canvas.height - b.r) {
            b.y = canvas.height - b.r;
            b.vy *= -1;
        }
    }

    function attract(p) {

        var dx = (p.x - X),
            dy = (p.y - Y),
            dist = Math.sqrt(dx * dx + dy * dy),
            angle = Math.atan2(dy, dx);

        if (dist > 10 && dist < 300) {
            if (!mousedown) {
                p.vx -= (20 / (p.r * dist)) * Math.cos(angle);
                p.vy -= (20 / (p.r * dist)) * Math.sin(angle);
            } else if (mousedown) {
                p.vx += (250 / (p.r * dist)) * Math.cos(angle);
                p.vy += (250 / (p.r * dist)) * Math.sin(angle);
            }
        }

    }

    function draw() {
        var p;
        for (var i = 0; i < P.length; i++) {
            p = P[i];

            if (mouseover) attract(p);
            bounce(p);

            p.x += p.vx;
            p.y += p.vy;

            p.vx *= .975;
            p.vy *= .975;

            ctx.fillStyle = p.col;
            ctx.fillRect(p.x, p.y, p.r, p.r);
            //ctx.beginPath();
            //ctx.fillStyle = p.col;
            //ctx.arc(p.x, p.y, p.r, 0, 2 * pi);
            //ctx.fill();


        }
        ctx.strokeStyle = (!mousedown) ? "rgba(255,255,255,1)" : "rgba(255,0,0,1)";

        ctx.beginPath();
        ctx.moveTo(X, Y - 10);
        ctx.lineTo(X, Y + 10);
        ctx.moveTo(X - 10, Y);
        ctx.lineTo(X + 10, Y);
        ctx.stroke();

    }

    function loop() {
        bg();
        draw();

        window.requestAnimationFrame(loop);
    }

    window.onresize = size;

    window.onmousemove = function(e) {
        X = e.clientX;
        Y = e.clientY;
    }

    window.onmousedown = function() {
        mousedown = true;
    }

    window.onmouseup = function() {
        mousedown = false;
    }

    var mouseover = false;

    window.onmouseover = function() {
        mouseover = true;
    }

    window.onmouseout = function() {
        mouseover = false;
    }

    init();
    loop();
}